1. Field of the Invention
The present invention relates to suppression of fire by extremely fine droplet water mist and more particularly, but not by way of limitation, to an improved method and apparatus for producing an extremely fine sub-micron size water mist using an electronic ultrasonic device that produces the mist at ambient-pressure and delivering the mist for application in suppressing fire.
2. Description of the Prior Art
Water based fire suppression systems have been in existence for many years. However, such systems were mostly replaced and the technology forgotten because of the advent of halon gas systems in the 1960's. In recent years, it has been discovered that halon gas is not environmentally safe, and its continued use has been banned due to its alleged potential to deplete ozone in the atmosphere. Thus, there is an urgent need for an alternative fire suppression system, which is effective and environmentally friendly and safe to use.
Because of several favorable properties, water mist has been reconsidered as a potential agent to replace halon gas. Water is environmentally friendly with no known toxic properties. Water has a specific heat of 4.18 J/g, and a high latent heat of vaporization of 2260 J/g that assist in cooling a flame. Finally, water is readily available and cost efficient.
Water mist suppresses fire through different mechanisms. Each mechanism exhibits a different degree of influence on the overall suppression efficiency of a water mist. The four important operating mechanisms are heat extraction, oxygen displacement, radiant heat attenuation, and dilution of the vapor/air mixture. Heat extraction and cooling of the flame has the maximum effect on the efficiency of fire suppression and the other mechanisms usually supplement the heat extraction mechanism. The inventors have found through computer simulation and experimentation that the success of water mist in its application to fire suppression depends on the ability to produce nearly nanometer-scale and sub-micron size droplets of water mist and deliver the mist to various fire scenarios. Extremely small droplets vaporize instantaneously and absorb energy to extract heat from the flame. Water mist droplets of larger diameters vaporize more slowly and are not as efficient in suppressing fires. Also, larger droplets are not as easily entrained into the fire and need additional momentum if the mist has to be introduced away from the firebase.
An extremely small amount of water is needed for suppressing a fire using extremely small sub-micron droplet mist because of considerable volume expansion accompanied by the transition from liquid state to mist (about 1700 times). This water expansion is based on the ratio of the density of liquid water and the gas-like nanoscale mist.
An extremely fine mist of sub-micron size water droplets avoids several of the disadvantages normally associated with the conventional water mist fire suppression technology. For instance, typical water mist applications having larger droplet size may cause a kinetic effect on flames causing flare-up from the water droplets striking the fuel surface. Further, because of slower vaporization and greater momentum needed, larger droplets wet surfaces within the area of application, conduct electricity and often damage items. Thus, a key to the success of water mist technology is the use of very fine nanometer-scale sub-micron water mist produced using a cost-effective and ambient-pressure method.
Previously, fine water mist production for fire suppression has been an expensive technology in terms of installation and maintenance. These prior art systems have included one or more expensive components such as high pressure storage of fluids, conduit pipes often under high pressure, and pumps providing pressurized fluid to specialized atomizer nozzles. Besides the expense of the components these components and conduit piping require valuable space for installation. Space may be limited for certain applications such as marine vessels, machine space, and computer data centers.
In addition to the expense of installing known water mist fire suppression systems, these systems present safety and mechanical concerns. In particular, pressurized systems are subject to leaks and hazards of bursting posed by retaining fluids under pressure. These systems require nozzles that are subject to clogging because of the small nozzle diameters and are also expensive and difficult to construct because of their precise specifications.
Even with state-of-the-art mechanical atomizers, the droplet size obtainable in these prior art systems is on the order of 50–200 microns. For many applications, these droplets are effective in cooling the flame. However, the water mist droplets may still wet surfaces and cause electrical conductance. This limits the ability to use water mist fire suppression in computer and data center applications or in precious item preservation rooms in libraries and museums. Moreover, the mechanical atomization technology required by conventional fine water mist fire suppression systems is still very expensive.
The prior art mist generation methods for fire suppression involve well-documented methods such as pressurized water or twin-fluid atomizers. Single fluid pressure based atomizers use water stored or pumped at high pressure (40 to 200 bar) and spray nozzles with relatively small orifice sizes. Twin-fluid systems use air, nitrogen, or other gases to atomize water at a nozzle. Although rare, there are some references to utilization of extremely high (hypersonic velocity) gas streams to generate ultrasonic waves to generate mist for suppressing fires and explosions. For instance, U.S. Pat. No. 4,378,851 to Egbert deVries describes ultrasonic nozzles of a general type in which a gas orifice penetrates a liquid filming surface. The method uses a high velocity gas stream to shear the thin layer of liquid and atomizing it. Others, U.S. Pat. Nos. 5,211,336 and 5,323,861, teach a method of producing a mist using a compressed air stream, and U.S. Pat. No. 5,597,044 teaches using a carrier gas having supersonic velocity. All the prior methods use either pressurized water or compressed gas as means of atomizing water to produce a water mist. As a result, these prior technologies produce atomized water mist using mechanical means that are not user friendly and are not very economical for generating water mist for fire suppression.
Thus, an objective of this invention is to provide a water mist fire suppression method using an electronic ultrasonic device to produce a water mist having sub-micron diameter water droplets.
Another objective of the invention is to provide a fire suppression device using an electronic ultrasonic device to produce a water mist and optionally powered by line fed electric power or a portable power source such as a battery.
Another objective of the invention is to provide a fire suppression method using a mist generation method that does not need pressurized water or gas.
Another objective of the invention is to use a method of generating mist for fire suppression that does not use an atomizing nozzle and is free from nozzle clogging and flow blockage.
Another objective is to provide a device and method to deliver a sub-micron diameter mist to a fire such that the mist that is entrained by the fire.
Another objective is to provide a mist for fire suppression without mechanically imparting excessive momentum to the mist.
Another objective is to provide a mist for fire suppression in which the mist is introduced from the base of the fire.
Another objective is to minimize water usage and the quantity of mist needed to suppress a fire by delivering the mist to the most reactive zone in the fire base using very low injection velocity.
Another objective is to reduce the quantity of water needed for suppressing a fire by several orders of magnitude compared to conventional mists by using water mist having submicron diameter droplets.
Another objective is to deliver a sub-micron mist to a fire such that the mist will vaporize before impact with surface areas and not wet surface areas or equipment.
Another objective is to provide a tangential flow of air or gas for carrying the mist out of the mist generator without affecting the centerline mist producing water fountain.